This section provides background information to facilitate a better understanding of the various aspects of the disclosure. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
It is desirable to measure or “log,” as a function of depth, various properties of earth formations penetrated by a borehole while the borehole is being drilled, rather than after completion of the drilling operation. It is also desirable to measure various drilling and borehole parameters while the borehole is being drilled. These technologies are known as logging-while-drilling (“LWD”) and measurement-while-drilling (“MWD”), respectively. Measurements are generally taken with a variety of sensors mounted within a drill collar above, but preferably close, to a drill bit which terminates a drill string. Sensor responses, which are indicative of the formation properties of interest or borehole conditions or drilling parameters, are then transmitted to the surface of the earth for recording and analysis.
The most common technique used for transmitting MWD data utilizes drilling fluid as a transmission medium for acoustic waves modulated downhole to represent sensor response data. The modulated acoustic waves are subsequently sensed and decoded at the surface of the earth. One type of telemetry device is a rotary valve or “mud siren” pressure pulse generator which repeatedly interrupts the flow of the drilling fluid, and thus causes varying pressure waves to be generated in the drilling fluid at a carrier frequency that is proportional to the rate of interruption.